1. Field of the Invention
The present invention relates to an optical head, a magneto-optical head, a disk apparatus, and a manufacturing method of the optical head, and in particular, relate to an optical head, a magneto-optical head, and a disk apparatus, that have high light efficiency, can realize a high-density recording medium and perform high-speed recording and reproduction, and can prevent erroneous record or erroneous reproduction, and a manufacturing method of the optical head.
2. Discussion of the Related Art
Recently, in order to perform high-density recording on a magneto-optical disk or a magnetic disk which records data with light and a magnetic field, and an optical disk which records data only with light, minimization of near field light spot used for recording or reproduction has been investigated.
As conventional disk apparatuses using this miniaturized near field light, there are what are shown in, U.S. Pat. No. 5,883,872, Japanese Patent Laid-Open No. 11-176007 (1999), and a reference xe2x80x9cApplied Physics Letters, vol. 65(6), p. 658(1994)xe2x80x9d.
FIG. 20 shows a disk apparatus shown in the above-described U.S. Pat. No. 5,883,872, and Japanese Patent Laid-Open No. 11-176007. This disk apparatus 1 has a laser source 2 emitting a laser beam, an objective lens 5 converging the laser beam emitted from the laser source 2, a solid immersion lens (SIL) 6 condensing the converged beam from the objective lens 5 and forming a light spot 9 on a light-condensed surface 6b of a bottom surface, a shade 7 that is provided on the light-condensed surface 6b of the SIL 6 and has a micro aperture 7a with the size smaller than that of the light spot 9, a beam splitter 13 separating the reflected light, derived from light emitted to a disk 12 through the micro aperture 7a, from emitted light from the laser source 2, and a photo detector 15 detecting the reflected light separated by the beam splitter 13. This aperture 7a is formed by coating the shade 7 on an entire surface of the light-condensed surface 6b of the SIL 6 and thereafter milling the shade 7 by using a focused ion beam method.
In the disk apparatus 1 configured in this manner, a laser beam emitted from the laser source 2 is converged by the objective lens 5 to be condensed on the light-condensed surface 6b of the SIL 6. Since the size of this aperture 7a is sufficiently smaller than that of the light spot 9, propagation light does not pass through this aperture 7a, and hence only near field light 10 leaks out on a surface of the aperture 7a of the light-condensed surface 6b. When a recording layer of the disk 12 is brought close to this near field light 10, this near field light 10 propagates into the recording layer, then recording and reproduction of information is performed. Since the size of the near field light 10 is determined with the size of the aperture 7a, minute recording and reproduction light that is a fraction of one or smaller than the size by only the SIL 6 can be obtained. Therefore, it is possible to increase recording density by using this for recording.
FIG. 21 shows a near field light microscope disclosed in the above-described reference. This near field light microscope 80 uses the near field light, whose light intensity is increased by plasmon resonance, for observation of a minute substance. This microscope 80 has an argon ion laser 83 emitting a blue laser beam 83a in an oblique direction, a hemispherical lens (SIL) 81 condensing the blue laser beam 83a, emitted from the argon ion laser 83, in a central part of a light-condensed surface 81a of a bottom surface, a micro metal particle 82 made of Ag with the diameter of 30 nm that is coated on the central part of a flat surface 81b of the hemispherical lens 81, and a photo multiplier (PM) 89 detecting reflected light 87 from an optical disk 125 through an objective lens 88. In the near field light microscope 80 configured in this manner, the blue laser beam 83a is made to enter a hemispherical incident surface 81a of the hemispherical lens 81 from an oblique angle so that the blue laser beam 83a emitted from the argon ion laser 83 is totally reflected on the flat surface 81b of the hemispherical lens 81. Furthermore, the blue laser beam 83a is condensed at and emitted to a position of a micro metal particle 82. Then, the plasmon resonance is generated in the micro metal ball 82, and near field light 84 generated therefrom is made to enter a recording film 86 of the optical disk 125. Moreover, reflected light 87 from the recording film 86 is condensed on the PM 89 by the objective lens 88 on the hemispherical lens 80 and is detect. In addition, the optical disk 125 is scanned in the X-Y direction with a piezo element, and recording marks in the recording film 86 are displayed by inputting an output signal of the PM 89 into a luminance signal of a monitor TV (not shown) in performing synchronization with the scanning. Although being a near field light microscope, this device can also be used for optical recording. Since it is possible to obtain the near field light 84 with the minute size, which is a fraction of the size in the case of only the hemispherical lens 81; it is possible to increase recording density by using this for recording.
FIG. 22 shows a metal structure described in the Dig. of the 6th Int. Conf. on Near-Field Optics and Related Tech. 2000, No. MoOI3 (2000). As shown in FIG. 22, the metal structure consists of small metal bodies 91a and 91axe2x80x2 faced each other with a small gap 91c between them. The width of apexes 91b and 91bxe2x80x2 of the metal bodies and the gap 91c are about 20 nm and far less than the wavelength of incident laser beam 92.
By arranging the polarization direction of the incident laser beam 92 to cross over the gap, a surface plasmon is excited in the metal bodies 92a and 92axe2x80x2 and vibrated in the direction parallel to the polarization direction, and electric charges having opposite polarities with each other in the apexes 92b and 92bxe2x80x2 causes dipole and the dipole generates the plasmon effectively. The induced electric charges which constitute an electric dipole, generate a strong near-field light 93 effectively, the size of which is nearly equal to that of the gap 92c. 
The simulation result shows that the dipole excited emits a near-field light which intensity is 2300 times larger than that of the incident light and is emitted only around the gap 91c. An experimental result about micro wave radiation with a dipole antenna (R. D. Grober et al.: Appl. Phys. Lett, Vol.70, No.11, (1997) p.1354) shows that the radiation occurs only around the gap region. The reason is that the antenna acts as a shield for the incident microwave because the conductivity of the metal antenna is so high enough to induce a strong dipole and the dipole has a strong shield effect.
But in the case of the optical frequency region (FIG. 22), the most of the incident wave passes side of the metal shade without coupling to the metal shade and is emitted out from the bottom surface of the transparent condensing medium, because the conductivity of the metal shade is not high enough to shield the incident wave, and the spot size of the incident is fairly larger than the size of the metal shade and its gap. The passed beam 92b irradiates and affects a recording medium when the medium is placed just under the metal bodies 92a and 92a for applying the near-field light for recording, which prevents the near-field light to make small recorded marks even if the size of the near-field light could be small enough.
According to a conventional disk apparatus shown in FIG. 20, light contributing to recording and reproduction is only the near field light 10 leaking out from the micro aperture 7a, and most of a laser beam condensed on the light-condensed surface 6b is reflected. In order that the shade 7 sufficiently shields a laser bear, usually, the thickness of more than several ten nm is necessary. Since the leak-out length of the near field light is generally in the same order to the size of an aperture, the diameter and depth of the aperture 7a becomes nearly equal if the diameter of the aperture 7a is stopped down at nearly 50 nm. Because of these reasons, the conventional disk apparatus has problems that the near field light entering a recording layer is significantly reduced in comparison with the laser beam condensed on the light-condensed surface 6b even if the recording layer of the disk 12 is brought close to the aperture 7a, and light efficiency is low, and hence it is not possible to increase transfer rates of recording and reproduction. In particular, at the time of reproduction, it is necessary to make the reflected light from the disk 12 enter the photo detector 15 through the micro aperture 7a. Nevertheless, since diffusing isotropically, the reflected light has a large flare angle, and hence a returning rate to this micro aperture 7a is low. In addition, if the intensity of the laser beam is increased, the laser beam absorbed by the shade 7 increases, and hence the shade 7 is heated and may be melted. Therefore, there is a problem that, consequently, it is not possible to achieve high-density and high-speed recording and reproduction.
In addition, if the conventional near field light microscope shown in FIG. 21 is used for recording of an optical disk, there are the following problems. In the conventional near field light microscope, the laser beam 83a is entered into the hemispherical lens 81 from an oblique angle since it is necessary to make the laser beam 83a be totally reflected on the light-condensed surface 81b so as to generate the plasmon resonance. Primarily, the diameter of the spot obtained is 10 xcexcm, and hence this is 3000 times or more as large as 30 nm of diameter of the micro metal member 82. Since only the partial light of this light spot that illuminates the micro metal member 82 contributes to the plasmon resonance, the light efficiency is extremely low. Therefore, there is another problem that a large photomutiplier is required for detecting reproduced light. In addition, since the near field light leaks out from a position of the light spot of the light-condensed surface 81b, this light becomes far stronger than the near field light generated from plasmon excited in the micro metal member 82. Therefore, there is still another problem that erroneous record or reproduction is performed by this light.
In addition, Goto et al. (The 73rd Micro-optics joint study group material, pp. 27-33) proposes a recording head having an ultra micro aperture, having a direct tapered-light guide part and an ultra micro coaxial tapered-wave guide in its end, in an end of output part of a surface emitting laser. Nevertheless, this is configured so that a beam is stopped down by making only a component which is totally reflected on a tapered surface before a coaxial part reaches the center of the coaxial part and making other components be cut. Therefore, as for light distribution of the beam reached, not only the distribution of light in a central part itself is few, but also the quantity of the light around the center is further weaker, the diameter of the beam inevitably becomes small. Therefore, when the beam enters the coaxial part, most of the beam is vignetted in the end of a central conductor, and hence only the residual weak light leaks out from its vicinity, and this is used as propagation light. Therefore, it is inferred that it is difficult to increase the transfer rates.
Therefore, the present invention provides an optical head, a magneto-optical head, and a disk apparatus, that have high optical efficiency, can realize a high-density recording medium and perform high-speed recording and reproduction, and can prevent erroneous record or erroneous reproduction, and a manufacturing method of the optical head.
According to the present invention, an optical head includes a transparent condensing medium which has a condensed surface to condense the laser beam to form a beam spot on the condensed surface, a shade provided on the transparent condensing medium and having an aperture, and a micro metal member at least part of which is positioned in the aperture. The aperture is positioned at which the beam spot is formed and the area of the aperture is smaller than the size of the beam spot.
According to the above configuration, by condensing light from a medium side of the transparent condensing medium, a light-condensing spot is minimized in inverse proportion to a refractive index of the transparent condensing medium. Therefore, it is possible to obtain a light spot that is highly efficient and minute in comparison with light condensed in the atmosphere. By illuminating the aperture, which has the micro metal member inside of the aperture, with the condensed laser beam, it becomes possible to perform recording and reproduction with high light efficiency.
Thus, the present invention uses scattering of a near field light that is formed on a light-condensed surface of a transparent condensing medium. The scattering is caused by the micro metal member, and the near field light that is strengthened by surface plasmon excitation in the micro metal member. By locating the near field light obtained by this surface plasmon excitation close to a recording medium, the near field light enters the recording medium to become propagation light, and makes it possible to perform recording and reproduction with high light efficiency. In addition, since the breadth of this near field light is nearly the size of the micro metal member, it becomes also possible to miniaturize the size of a recording mark by miniaturizing the micro metal member, so it becomes possible to perform high-density recording. Furthermore, in the present invention, a partial laser beam that is condensed outside the aperture does not enter the recording medium since being cut off by the shade, so it is possible to prevent erroneous record or erroneous reproduction. Moreover, at the time of reproduction, since the reflected light from the disk is made to enter through the comparatively large doughnut-shaped aperture surrounding the micro metal member in the central part, it becomes also possible to efficiently bring in the reflected light.
Another aspect of the present invention provides an optical head including a transparent condensing medium which has a condensed surface and condenses the laser beam to form a beam spot on the condensed surface, a shade provided on the transparent condensing medium and having an aperture positioned at which the beam spot is formed, the area of the aperture being smaller than the size of the beam spot, and a micro metal member at least part of which is positioned in the aperture. The shade and the micro metal member have the thickness of one-half or larger of a wavelength of the laser beam in the transparent condensing medium.
This aspect is characterized in that the propagation light from the aperture can be efficiently used for recording and reproduction. First, a spot having high intensity by being condensed on a light-condensed surface is emitted to an aperture having coaxial structure. Thus, by making the shade of metal and making its thickness comparatively thick, that is, one-half or thicker of a wavelength of the laser beam in the medium, the laser beam around it becomes a wave in a mode where an electric field and a magnetic field are in parallel with the metal shade and do not have a component in the propagating direction, that is, a TEM (Transverse Electromagnetic) wave. Since a wave in this mode does not have a cutoff wavelength, the wave can pass through a doughnut-shaped aperture, which is narrower than the wavelength, and to propagate without loss in principle. Furthermore, when the light enters a coaxially structural part, the spot diameter of the light spot formed by condensing the light is larger than the size of the micro metal member and the spot has wide light quantity distribution. Therefore, it is possible to supply sufficient quantity of light to the coaxially structural part. Hence, it is possible to efficiently obtain the light spot, having high intensity, from the micro aperture as a laser beam with the small spot size. Therefore it is possible to perform recording and reproduction that is efficient and has each high transfer rate. In addition, since it is possible to minimize the size of a recording mark, high-density recording becomes possible.
Furthermore, the size of the micro metal member in the present invention denotes an area of a cross-section that is perpendicular to the traveling direction of the light when the light enters the micro metal member. The shape of the micro metal member is not limited to a cylinder, but the shape can also be a polygon or a three-dimensional structure, etc.